New, energy-efficient stoves have been designed which utilize magnetic induction as the source of heat, rather than electrical resistance or gaseous combustion. Cooking utensils for use with such stoves have conventionally been made from ferromagnetic metals such as cast iron and magnetic steel. The mechanism of heating with such materials is predominantly resistance heating caused by eddy currents induced in the ferromagnetic metal. It has been recognized that another class of materials could also be useful in such an application. Such materials would contain a significant proportion of a ferrimagnetic crystal phase, which crystals respond thermally to fields generated by induction coils, the degree of response appearing to be not only directly related to the magnetic permeability of the crystal phase, but also being dependent upon the microstructure and volume percent of the crystal phase. The magnetic characteristics of the crystal phase serve to concentrate the magnetic flux from an induction source into the material. Thermal energy giving rise to heating effects results primarily from magnetic hysteresis and dielectric and resistive losses; hence, a different heating mechanism from that of the ferromagnetic metal. Magnetite is deemed to be the preferred ferrimagnetic crystal phase.
U.S. Pat. No. 4,083,727 discloses the manufacture of glass-ceramic articles having compositions within the Li.sub.2 O-Al.sub.2 O.sub.3 -SiO.sub.2 -TiO.sub.2 system wherein the predominant crystal phase is beta-quartz solid solution and/or beta-spodumene solid solution, but which have a thin, integral surface film containing crystals exhibiting the structure of magnetite. The method of that invention contemplates two basic steps. (1) A glass article having a particular composition is heat treated in an oxidizing atmosphere to yield a glass-ceramic article containing beta-quartz solid solution and/or beta-spodumene solid solution as the predominant crystal phase in the interior portion of the article. Also during that heat treatment, a thin surface layer is developed in situ on the glass-ceramic article containing hematite crystals. (2) The glass-ceramic article is heat treated in a H.sub.2 O-containing reducing environment to convert the hematite crystals in the surface layer to magnetite. The compositions of those articles consist essentially, in weight percent on the oxide basis, of about 1-6% FeO, 3-10% Li.sub.2 O, 15-40% Al.sub.2 O.sub.3, 40-75% SiO.sub.2, and 2-7% RO.sub.2, wherein RO.sub.2 consists of 2-6% TiO.sub.2 and 0-3% ZrO.sub.2. FeO levels above 6% are eschewed as leading to cracking, crazing, and dicing of the articles.
U.S. Pat. No. 4,084,973 describes the production of glass articles having compositions within the R.sub.2 O-Al.sub.2 O.sub.3 -SiO.sub.2 field, wherein R.sub.2 O consists of Li.sub.2 O, Na.sub.2 O, and/or K.sub.2 O. When the glasses are heat treated in an oxidizing environment, a thin surface layer containing hematite crystals (alpha-Fe.sub.2 O.sub.3) is developed in situ. The hematite crystals can be subsequently reduced to magnetite crystals via heat treatment in a H.sub.2 O-containing environment.
The compositions of those glasses consist essentially, in weight percent on the oxide basis, of about 1-15% R.sub.2 O, where R.sub.2 O consists of 0-10% Li.sub.2 O and 0-15% Na.sub.2 O and/or K.sub.2 O, 0.3-13% FeO, 15-35% Al.sub.2 O.sub.3, 55-80% SiO.sub.2, and 0-5% TiO.sub.2 and/or ZrO.sub.2. The presence of TiO.sub.2 is stated to control the in situ development of the surface crystallization such that a thin, mirror-like film development is favored. Further, the inclusion of TiO.sub.2 and/or ZrO.sub.2 is declared to improve the thermal stability of the surface crystalline films.
Heat treatment temperatures of 675.degree.-950.degree. C. are utilized to develop the surface crystallization of hematite. Higher temperatures are cautioned as hazarding the growth of crystals in the interior of the glass having low coefficients of thermal expansion, e.g., beta-quartz solid solution and/or beta-spodumene solid solution.
U.S. Pat. No. 3,193,503 discloses the production of glass-ceramic articles consisting essentially, by weight, of 16-50% MgO, 37-60% Fe.sub.2 O.sub.3, 20-45% SiO.sub.2, and 0-15% of mineralizers or nucleants such as CaF.sub.2, CoO, NiO, V.sub.2 O.sub.5, MoO.sub.3, and ThO.sub.2. The resultant articles are termed "magnetic ceramic ferrites" but no crystallization identification data are provided.
U.S. Pat. No. 3,694,360 describes the manufacture of glass-ceramics exhibiting ferrimagnetic properties. The compositions therefor consist essentially, in parts by weight, of 35-55% Fe.sub.2 O.sub.3, 5-15% Li.sub.2 O, 10-50% SiO.sub.2, and 1-15% ZnO. The predominant crystal phase is lithium ferrite.
U.S. Pat. No. 3,741,740 is concerned with the formation of glass-ceramic articles from glasses consisting essentially, by weight, of 3.5% Li.sub.2 O, 0.6-5% Fe.sub.2 O.sub.3, 18-22% Al.sub.2 O.sub.3, 60-70% SiO.sub.2, and 2-7% of a nucleant selected from the group TiO.sub.2, ZrO.sub.2, SnO.sub.2, P.sub.2 O.sub.5, and Cr.sub.2 O.sub.3. Upon heat treatment of the precursor glass article in a non-reducing atmosphere, a high quartz solid solution phase is developed in the interior portion of the article and an integral reflective surface film is formed. Neither the composition nor the microstructure of the surface film is disclosed.
U.S. Pat. No. 3,492,237 describes glass-ceramic articles having compositions within the Li.sub.2 O-Na.sub.2 O-Al.sub.2 O.sub.3 -Fe.sub.2 O.sub.3 -SiO.sub.2 field wherein lithium ferrite is a primary crystal phase. The articles have a mole ratio of SiO.sub.2, Na.sub.2 O, and Al.sub.2 O.sub.3 of 11-13:3-4:4-1 with 1-10 moles each of Fe.sub.2 O.sub.3 and Li.sub.2 O per mole of Al.sub.2 O.sub.3. Crystals of acmite and possibly albite are also present with the lithium ferrite.
U.S. Pat. No. 3,503,763 is directed to devitrifying solder glasses which crystallize in situ to produce crystals of beta-eucryptite or beta-spodumene. The compositions of the glasses consist essentially, by weight, of 13-23% PbO, 4-11% B.sub.2 O.sub.3, 4-6% Li.sub.2 O, 14-19% Al.sub.2 O.sub.3, 39-50% SiO.sub.2, 1-6% TiO.sub.2 and/or 1-3% ZrO.sub.2, and 2-5% iron oxide when the amount of B.sub.2 O.sub.3 is 4-6%. The iron oxide is stated to aid nucleation and is used to replace part of the B.sub.2 O.sub.3 because of its strong fluxing action.
U.S. Pat. No. 3,929,497 is drawn to the preparation of glass-ceramic fibers consisting essentially, by weight, of 5-15% Na.sub.2 O, 5-15% Fe.sub.2 O.sub.3, 5-15% MgO, 10-15% CaO, 10-15% Al.sub.2 O.sub.3, 45-50% SiO.sub.2, and 0.5-5% TiO.sub.2. X-ray diffraction analyses identified diopside as the principal crystal phase and magnetite was not found.
U.S. Pat. No. 3,962,514 discloses glass-ceramic articles having exuded transition metal spinel surface films thereon. The base compositions therefor consist essentially, by weight, of 14-35% Al.sub.2 O.sub.3, 55-80% SiO.sub.2, 0-5% Li.sub.2 O, 3-13% RO.sub.2, wherein RO.sub.2 consists of 0-7% TiO.sub.2 and 0-10% ZrO.sub.2, 0-3% F, and 0.1-10% transition metal oxides selected from the group of 0.5% MnO.sub.2, 0-5% Fe.sub.2 O.sub.3, 0-3% CoO, 0-2% CuO, 0-2% Cr.sub.2 O.sub.3, 0-3% V.sub.2 O.sub.5, and 0-10% NiO. Upon being crystallized in situ, the glasses are converted into glass-ceramic articles containing beta-quartz or beta-spodumene type crystals as the principal phase. The glass-ceramic articles are thereafter heat treated under reducing conditions to cause the exudation of a transition metal compound exhibiting a spinel structure. Such compounds included Mn.sub.3 O.sub.4, Fe.sub.3 O.sub.4, NiAl.sub.2 O.sub. 4, CoAl.sub.2 O.sub.4, CuCr.sub.2 O.sub.4, MnCr.sub.2 O.sub.4, CrAl.sub.2 O.sub.4, Co.sub.3 O.sub.4, FeCr.sub.2 O.sub.4, CoFe.sub.2 O.sub.4, MnFe.sub.2 O.sub.4 and CoMn.sub.2 O.sub.4. Those crystal phases are not reported as being present in the interior of the article.
U.S. Pat. No. 3,926,602 describes the production of glass-ceramic articles having beta-quartz solid solution or beta-spodumene solid solution crystals in the interior portion of the articles and an integral surface layer exhibiting a reflective metallic luster consisting of hematite (alpha-Fe.sub.2 O.sub.3) crystals dispersed in a glassy matrix. The base composition therefor consist essentially, by weight, of 0.5-3.5% FeO, 3-6% Li.sub.2 O, 16-21% Al.sub.2 O.sub.3, 65-75% SiO.sub.2, and 1.5-7% RO.sub.2, wherein RO.sub.2 consists of 1.5-6% TiO.sub.2 and 0-3% ZrO.sub.2.
It will be appreciated that, as a matter of convenience, the above patents report the total iron oxide content of the inventive materials, present as a combination of FeO and Fe.sub.2 O.sub.3, as either "FeO" or "Fe.sub.2 O.sub.3 ". Thus, for the sake of simplicity and because the materials were apparently not analyzed for the individual proportions of FeO and Fe.sub.2 O.sub.3, the full amount of the iron oxide content was expressed as either "FeO" or "Fe.sub.2 O.sub.3."